The implant of a prosthesis to replace a damaged or worn joint is a well-known practice. In particular, for the implant of a knee prosthesis it is known that a distal resection is performed on the patient's femur and subsequently a femoral finishing which comprises the execution of an anterior resection, a posterior resection, an anterior chamfer cut and a posterior chamfer cut. During execution of the femoral finishing, correct performance of the resections and chamfer cuts is a decisive factor for correct positioning of the prosthesis. Furthermore, the inclinations imparted to the bone resection surfaces and to the chamfer cuts determine postural corrections of the joint, for example the intra- or extra-rotation of the knee.
To carry out the femoral finishing, it is known that multipurpose metal instruments are used (also called 4-in-1 cutting jigs) which adapt to the distal femoral part and are configured to guide, in use, sickle blades. These metal instruments can be re-used several times on different patients and the correct positioning of the metal instruments substantially depends on the ability of the surgeon at the time of the operation. In particular, during the operation, the surgeon has to manually adjust the positioning of the instruments to obtain a desired intra- or extra-rotation of the joint.
The metal instruments of the type described above have the drawback of having to rely mainly on the ability of the surgeon during the operation and of not guaranteeing that the desired intra or extra rotations will be obtained. In particular, slight variations in the positioning of the metal instruments can cause errors with consequent problems in positioning of the prosthesis and in articulation of the knee.
Alternatively, to the solution described above, the distal resection is performed by means of a disposable instrument produced specifically on the basis of the patient's femoral morphology also called PSI, Patient Specific Instrument, for example obtained by means of magnetic resonance imaging, CAT scan or radiography. This instrument is configured also to provide reference holes on the distal bone resection surface which are used for positioning, by means of pins, a metal 4-in-1 cutting jig to guide the sickle blades for execution of the anterior and posterior resection and the anterior and posterior chamfer cuts.
This type of PSI allows a particularly accurate distal cut to be performed according to the femoral morphology of the patient. Furthermore, this instrument allows positioning of the 4-in-1 cutting jig to be guided via the positioning indication of reference pins for positioning said 4-in-1 cutting jig.
However, it is known in literature, for example from:                “Unsatisfactory Accuracy as Determined by Computer Navigation of VISIONAIRE Patient-Specific Instrumentation for Total Knee Arthroplasty”Sébastien Lustig MD, PhD, b, Corey J. Scholes PhD, Sam I. Oussedik FRCS, Vera Kinzel FRACS a, Myles R. J. Coolican FRACS, David A. Parker FRACSThe Journal of Arthroplasty 28 (2013)        “Patient-specific instrumentation for total knee replacement verified by computer navigation: a case report”Kok-Yu Chan, Yee-Hong Teo, Department of Orthopaedic Surgery, Tan Tock Seng Hospital, SingaporeJournal of Orthopaedic Surgery 2012; 20        “Patient-specific Total Knee Arthroplasty Required Frequent Surgeon-directed Changes”Benjamin M. Stronach MD, Christopher E. Pelt MD, Jill Erickson PA, Christopher L. Peters MDClin Orthop Relat Res (2013)        “Evaluation of the accuracy of a patient-specific instrumentation by navigation”Fabio Conteduca, Raffaele Iorio, Daniele Mazza, Ludovico Caperna, Gabriele Bolle, Giuseppe Argento, Andrea Ferretti Knee Surg Sports Traumatol Arthrosc, 21(2013)        “Component Alignment During Total Knee Arthroplasty with Use of Standard or Custom Instrumentation”Steven T. Woolson, MD, Alex H. S. Harris, PhD, David W. Wagner, PhD, and Nicholas J. Giori, MD, PhDTHE JOURNAL OF BONE AND JOINT SURGERY, 2014that incorrect positioning of the PSI distal cutting jig due to incorrect preoperative planning, interference with osteophytes on the femoral surface or simply due to positioning difficulty (the femoral surface is convex, the surface of the PSI jig is concave and there are no particular and easy references for positioning) results in incorrect indication of the references for subsequent positioning of the 4-in-1 jig which determines correct intra- or extra-rotation of the knee.        
In order to obtain a desired intra- or extra-rotation of the knee, the surgeon has to manually adjust the inclination of the 4-in-1 cutting jig. Therefore, correct positioning of the 4-in-1 cutting jigs of known type, i.e. correct inclination of the 4-in-1 cutting jig on the distal bone resection surface, relies on the ability of the surgeon during the operation.
Therefore, the known 4-in-1 cutting jig of the type described above has the drawback of not guaranteeing correct positioning to obtain a given intra or extra rotation, with consequent problems, in the event of error, in positioning of the prosthesis and articulation of the knee. Furthermore, it has been found that errors in positioning of the disposable instruments for performance of the distal resection can occur and the 4-in-1 cutting jigs of the type described above do not allow any errors in execution of the distal resection to be indicated and if necessary corrected.